1. Field of Invention
This invention pertains to a proportion and control valve for a suspension assembly associated with an air braking system for a tractor/trailer vehicle. The invention is particularly applicable to vehicles such as trucks/tractor systems known as a 6.times.2 vehicle employing a traction control system with air suspension control transfer. It will be appreciated, however, that the invention may have broader applications and may be advantageously employed in related environments or applications.
2. Description of Related Art
By way of brief background, a 6.times.4 truck and tractor system employs a pair of drive axles. As will be appreciated, the vehicle cost associated with a pair of drive axles is substantially greater than a 6.times.2 arrangement, i.e., a system in which only one of the two rear axles is a drive axle, because of the additional drive components. Since a 6.times.4 system has increased weight, operating costs, complexity, maintenance costs, friction, and fuel consumption associated therewith, a 6.times.2 assembly is highly desirable. On the other hand, a 6.times.2 system has decreased traction capabilities relative to a 6.times.4 configuration. Accordingly, a 6.times.2 system using an air suspension control to transfer load to the drive axle has been proposed as a preferred arrangement that achieves enhanced traction control.
Generally, traction control systems employ similar principles to antilock braking systems on wet or slippery surfaces, curves, split surfaces, ice, and the like, traction control assemblies sense when the wheels of a vehicle spin upon acceleration. This is representative of a loss of traction between the road surface and the tire. In response, drive torque is transferred to the non-spinning tires or wheels and a braking force is gently applied to the spinning wheel. This transfers the torque through the differential to the non-spinning, or more slowly spinning wheel. If both wheels spin, then the engine RPM is electronically controlled and reduced to an appropriate level.
As briefly indicated above, it has been proposed to use a shift or transfer the vehicle load in a 6.times.2 arrangement. An air bag suspension assembly associated therewith reduces the suspension bag pressure in the non-drive axle so that more weight is transferred to the drive axle. In this manner, an increased portion of the load is transferred to the drive axle to enhance traction.
Transferring all of the load to the drive axle can overload the drive axle in the laden mode. Moreover, transferring all of the load to the drive axle can bring into consideration axle warranty concerns and legal highway weight limits. Thus, a need exists for a proportional transfer of the load between the drive and non-drive axles in response to traction control events.